This invention relates to thermoplastic extrusion technologies. More particularly, this invention relates to extrusion of thermoplastic in a predetermined spatial pattern under computer control.
Thermoplastic extrusion technologies perform rapid prototyping of three-dimensional objects by selectively extruding a molten thermoplastic from an extrusion head while moving the extrusion head in three dimensions with respect to a base. The thermoplastic is extruded in "beads" or "roads" that solidify after being deposited. Movement of the extrusion head with respect to the base is performed under computer control, in accordance with design data provided from a computer aided design (CAD) system. U.S. Pat. Nos. 5,121,329 and 5,764,521, commonly assigned to Stratasys, Inc., the assignee of the present invention, describe the rapid prototyping deposition modeling technology and are hereby incorporated by reference as if set forth fully herein.
In existing thermoplastic extrusion technologies, the configuration of the extrudate is adjustable in quantity and flow rate but not in spatial configuration. The flow rate of material out of an orifice is carefully controlled, but the spatial configuration (e.g., road width) of the flow is not readily alterable.
As the size of the element of additive material shrinks, a prototype part built with those additive elements will typically represent its CAD model parent with greater fidelity. For example, depositing layers of thermoplastic half as thick using extruded roads that are half as wide will improve the feature detail and surface finish of a model by about a factor of two. Unfortunately, with a constant deposition velocity, the time to build the model with this factor of two increase in resolution increases by about a factor of eight.
This speed/resolution conflict has been resolved in other rapid prototyping technologies by replacing vector motion of a single source with raster motion of multiple ink jets. An example is the Actua.TM. ink jet rapid prototyping system from 3D Systems, Inc., which forms three-dimensional models from a wax-like material. The ink jets are individually controlled so that any number of the jets will deposit the modeling material at a given time. Ink jet-based technologies are attractive for extruding discrete quantities of relatively low viscosity materials, however, ink jetting techniques have difficulty with high viscosity materials (such as thermoplastics) and particulate or fiber-filled materials. These materials tend to clog the jets. Also, in thermoplastic extrusion, "wetting" of the base by the extruded thermoplastic serves to separate the thermoplastic from the extrusion head, while ink-jetted materials break free due to the jetting momentum.
There is an unmet need for a computer-controlled extrusion apparatus suitable for dispensing thermoplastic in an extrudate configuration that may be varied quickly during deposition, in accordance with movement of the apparatus in a predetermined spatial pattern relative to a base.